1. Field of the Invention
The present invention relates to a multiplexing optical system. Specifically, the present invention relates to a multiplexing optical system for combining laser beams emitted from a plurality of semiconductor lasers into a single beam by making the laser beams enter an optical fiber.
2. Description of the Related Art
Conventionally, multiplexing optical systems are well known, for example, as disclosed in Japanese Unexamined Patent Publication No. 2002-202442 and Japanese Unexamined Patent Publication No. 2003-158332. In the multiplexing optical systems, laser beams emitted from a plurality of semiconductor lasers are condensed by a lens system, and the laser beams converge at a point. Then, the plurality of laser beams is combined into a single laser beam by making the laser beams enter an optical fiber placed in such a manner that the surface of an incident end of the optical fiber is positioned at the convergence position of the laser beams.
More specifically, in the conventional technique disclosed in Japanese Unexamined Patent Publication No. 2002-202442, a lens system including a plurality of collimator lenses and a condensing lens formed by a single axially symmetric lens is used as the lens system. The plurality of collimator lenses collimates divergent laser beams that have been emitted from the semiconductor lasers. The axially symmetric lens condenses the laser beams transmitted through the collimator lenses.
In Japanese Unexamined Patent Publication No. 2003-158332, a broad-area laser is used as a semiconductor laser to obtain a combined laser beam that has higher output power. The broad-area laser is a laser, of which the stripe width (light emission width) is wider. The lens system used in Japanese Unexamined Patent Publication No. 2003-158332 is basically similar to the lens system used in Japanese Unexamined Patent Publication No. 2002-202442. When the stripe width of the semiconductor laser is W and the magnification of the lens system is M, the size of condensed light on the surface of an incident end of an optical fiber is MW in a direction corresponding to the stripe width. If a semiconductor laser, such as a broad-area laser, that has a wider stripe width W is used, the size MW naturally becomes larger. In some cases, the size MW exceeds the core diameter of the optical fiber, and the input efficiency of the laser beam with respect to the optical fiber becomes lower.
To solve such problems, a technique for reducing the size MW of condensed light has been proposed, as disclosed in Japanese Unexamined Patent Publication No. 2004-077779. In this technique, a plurality of semiconductor lasers is arranged on concentric circles and placed as close to each other as possible. Accordingly, it becomes possible to adopt a lens system with a lower magnification M, thereby reducing the size MW of the condensed light.
Further, a lens system, as disclosed in Japanese Unexamined Patent Publication No. 2002-048491, is well known as a lens system for condensing a laser beam emitted from a semiconductor laser. In the lens system, a cylindrical lens with magnification Ms and a cylindrical lens with magnification Mf are combined so that the magnifications satisfy Ms<Mf. The cylindrical lens with magnification Ms is a lens that has power only within a plane including the stripe width direction of the semiconductor laser (hereinafter, this direction is referred to as a “slow axis direction” for convenience). The cylindrical lens with magnification Mf is a lens that has power only within a plane including a direction perpendicular to the stripe width direction (hereinafter, the direction perpendicular to the stripe width direction is referred to as a “fast axis direction” for convenience). Accordingly, it is possible to prevent the size of condensed light of the laser beam from increasing in one direction.
The structure disclosed in Japanese Unexamined Patent Publication No. 2004-077779 may achieve an expected result. However, there is a problem that mounting is very complicated.
Further, the lens system disclosed in Japanese Unexamined Patent Publication No. 2002-048491 may be adopted in the multiplexing optical systems disclosed in Japanese Unexamined Patent Publication No: 2002-202442 and Japanese Unexamined Patent Publication No. 2003-158332. However, in that case, the shape of the condensed light of the laser beam becomes a shape with a tail extending toward the outside of the optical axis of the lens system, and a light condensing characteristic deteriorates. Further, a problem that the input efficiency of the laser beam with respect to the optical fiber becomes lower is recognized.
FIGS. 9A and 9B are schematic diagrams illustrating the tail generation phenomenon. In FIGS. 9A and 9B, a case in which eight semiconductors (2×4=8) are arranged next to each other is used as an example. At light emission positions of the semiconductor lasers (LD), laser beams B11, B12, B13 and B14 and laser beams B21, B22, B23 and B24 are arranged as illustrated in FIG. 9A. However, after the laser beams are condensed by two cylindrical lenses, the state of condensed light on a plane onto which the condensed laser beams are projected (the surface of the incident end of the optical fiber) becomes as illustrated in FIG. 9B. Specifically, if a laser beam is emitted from a semiconductor laser that is positioned on the outer side of the optical axis of the optical system, the degree of tail generation becomes higher. Therefore, it is difficult to efficiently make such laser beams enter an optical fiber with a core diameter of approximately 50 μm.